by Roger D. Launius, NASA Chief Historian
The twentieth of July 1996 marks the twentieth anniversary of Viking 1's touch down on Mars after a voyage of nearly one year, followed within two months by Viking 2. The landings represented the culmination of a series of missions to explore the planet Mars that had begun in 1964 with Mariner 4, and continued with the Mariner 6 and Mariner 7 flybys in 1969 and the Mariner 9 orbital mission in 1971 and 1972.
After failing to obtain approval for a more ambitious and expensive program to explore Mars in the late 1960s, NASA came forward with a somewhat more modest $1 billion budget for the Viking expedition to the Red Planet. This purchased tandem spacecraft designed to orbit Mars and to land and operate on the planet's surface. Two identical spacecraft, each consisting of a lander and an orbiter, were built. Launched on 20 August 1975 from the Kennedy Space Center, Viking 1 spent nearly a year cruising to Mars, placed an orbiter in operation around the planet, and landed on 20 July 1976 on the Chryse Planitia (Golden Plains). Viking 2 was launched on 9 September 1975 and landed on 3 September 1976. The Viking project's primary mission ended on 15 November 1976, 11 days before Mars' superior conjunction (its passage behind the Sun), although the Viking spacecraft continued to operate for six years after first reaching Mars. Its last transmission reached Earth on 11 November 1982. Controllers at NASA's Jet Propulsion Laboratory tried unsuccessfully for another six and one-half months to regain contact with the lander, but finally closed down the overall mission on 21 May 1983.
With a single exception--the seismic instruments--the scientific return from the expedition was spectacular. Unfortunately, the seismometer on Viking 1 did not work after landing, and the seismometer on Viking 2 detected only one event that may have been seismic. On the other hand, the two landers continuously monitored weather at the landing sites and found both exciting cyclical variations and an exceptionally harsh climate. Atmospheric temperatures at the more southern Viking 1 landing site, for instance, were only as high as +7 degrees Fahrenheit at midday, but the predawn summer temperature was -107 degree Fahrenheit. And the lowest predawn temperature was -184 degrees Fahrenheit, about the frost point of carbon dioxide. The project also observed the Martian winds, finding that they generally blew more slowly than expected.
One of the important scientific activities of this project was the attempt to determine whether there was life on Mars, since the planet had long been thought of as having sufficient similarity to the Earth that life might exist there. While the three biology experiments discovered unexpected and enigmatic chemical activity in the Martian soil, they provided no clear evidence for the presence of living microorganisms in soil near the landing sites. According to mission biologists, Mars was self-sterilizing. They concluded that the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil, and the oxidizing nature of the soil chemistry had prevented the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past, however, remains open.